Fuel cells convert the electrochemical energy of fuel oxidation reactions into electrical energy. PEM fuel cells are highly efficient devices that operate on hydrogen or on a variety of fuels that can be chemically converted to hydrogen for reaction in the fuel cell. For example, fossil fuels and bio-fuels can be chemically reformed to supply hydrogen. In general, hydrogen gas has been the preferred fuel because of its high reactivity for the electrochemical anode reaction and because the cathode oxidation reaction of the hydrogen ion produces water as a byproduct.
The simplest PEM fuel cells consist of an anode and a cathode sandwiched around an electrolyte. Frequently the electrolyte is a solid-state electrolyte, such as a solid polymer proton exchange electrolyte membrane. Hydrogen fuel is supplied to the anode and oxygen is supplied to the cathode. In the presence of a catalyst such as platinum, the hydrogen atom splits into a proton (hydrogen ion) and an electron at the anode. The proton and electron then proceed along separate paths to the cathode; while the proton reaches the cathode via the electrolyte the electron creates a separate current through an electrical circuit. The proton and electron reunite at the cathode and react with oxygen to produce water. Overall, the electrochemical reactions involved are:
At the anode:2H2 → 4H+ 4e−At the cathode:O2 + 4e− + 4H+ → 2H2OOverall:2H2 + O2 → 2H2O
In order to maximize the contact area available between the hydrogen fuel, the oxygen, the electrode, and the electrolyte, and in order to minimize the distance that the protons need to travel between the electrodes, the electrodes and electrolyte are usually made to be flat and thin. In addition, the structure of the electrodes is usually porous. However, selection of the composition, the porosity and the dimensions of the electrodes and electrolyte for optimal efficiency of the electrochemical reaction are frequently limited by need for structural integrity required for leak free control of fuel and oxygen at pressures and flow rates that optimize the overall power of the fuel cell.
The voltage produced between the anode and cathode of a fuel cell is typically on the order of about 0.7 V. As a consequence, in order to produce a practical voltage (e.g., between about 10 and 100 V) many fuel cells need to be connected in series referred to as a fuel cell “stack”. The preferred method of connecting neighboring fuel cells in a stack involves separating them with bipolar plates. The bipolar plates provide an electrical connection between the anode and cathode of neighboring fuel cells and provide a means of supplying hydrogen to the anode of one fuel cell and a means of supplying oxygen to the cathode of its neighboring cell.
As stated above, gaseous hydrogen is the preferred PEM fuel. However, employing gaseous hydrogen in PEM fuel cell technology poses several practical difficulties. In the accessible environment, hydrogen does not occur naturally in its elemental state, but must be generated either at the fuel cell location or remotely. When generated remotely, hydrogen fuel must be transported, stored and delivered to the fuel cell. At atmospheric pressure, the low energy density of gaseous hydrogen limits the theoretical power density of the fuel cell. To raise the energy density, higher pressures are used. One negative effect is that the heavy, bulky and expensive storage and delivery systems required to employ high pressure hydrogen gas systems produce low power to total weight ratios and limit application of the technology in many situations. Additionally, gaseous hydrogen's flammability poses significant safety concerns. Safety, weight and power restrictions make alternatives to stored hydrogen gas desirable.
One alternative is to employ available hydrocarbon compounds as primary fuels that can be chemically converted to produce gaseous hydrogen as a secondary fuel for fuel cell consumption. Different chemical conversion technologies are available to convert such fuels into gaseous hydrogen. For example, various fossil fuels may be catalytically reformed into hydrogen rich mixture. However, hydrocarbon fuels pose several difficulties. Reforming most hydrocarbons requires complex mechanical systems, additional catalysts and high temperatures. Undesirable environmental pollutants are common byproducts. Additionally, common byproducts of reforming can significantly reduce the efficiency of fuel cell catalysts. An interesting exception is methanol, which does not require reforming for use in PEM fuel cells. Methanol, mixed with water, can be catalyzed to produce hydrogen ions at the anode of the fuel cell. Unfortunately, such direct methanol fuel cells have slow rates of reaction and, thus, low operating voltages, low power density and low efficiency.
An emerging alternative approach in fuel cell technology is to employ, as primary fuels, hydrogen dense specialized chemical compounds as alternatives to stored gaseous (or liquid) hydrogen and to reformed hydrocarbon fuels. Critical features of these compounds are: (1) for their mass, the compounds hold large quantities of hydrogen; (3) the compounds easily release their hydrogen; (2) manufacture of the compounds is simple and requires little energy; (4) the compounds are stable and safe to handle. Additional desirable features for these compounds are that the compounds and their byproducts are non-toxic and recyclable.
While a large number of specialized chemicals have been investigated, the hydrides have been shown to the most important of these potential hydrogen storage compounds. Simple metal hydrides, such as alkali metal hydrides and rare earth metal hydrides, have been investigated. When reacted with water, alkali metal hydrides give off hydrogen gas. The rare earth metal hydrides, under the right conditions, simply release hydrogen gas without requiring another reactant. Complex hydrides, such as borohydrides and aluminum hydrides, have also been investigated. Unfortunately, many of these hydrides have one or more drawbacks, such as being toxic, caustic or have hydrogen release rates that are difficult to control at operating temperatures.
Although specialized chemicals have reduced the need to provide hydrogen gas storage and delivery systems, the current art does not completely eliminate the use of gaseous hydrogen in practical PEM fuel cells. Except for direct methanol feed fuel cells, all practical PEM fuel cells use hydrogen in a gaseous state within the anolyte fuel channels. Most of the limitations of employing gaseous hydrogen remain, including: a low energy density at atmospheric pressures; design compromises necessary to accommodate higher than atmospheric pressure hydrogen gas systems; lowering of power to total weight ratios; and hydrogen's flammability.
Accordingly, it would be desirable to provide a safe, non-toxic, recyclable fuel that has a high density of hydrogen and is readily useable in a PEM fuel cell without producing an accumulation of gaseous hydrogen in the anolyte fuel channels or other fuel supply systems.